Long-term changes in episodic acidification of streams in Shenandoah National Park, Virginia (U.S.A.)

Abstract

Episodic acidification-the short-term loss of acid neutralizing capacity (ANC) in surface waters during periods associated with rainfall and snowmelt runoff-has been shown in previous field studies to be a ubiquitous process that can have long-term adverse effects on fish populations. A systematic field study of episodic acidification in five gaged Shenandoah National Park (SNP) streams was initiated in 1991, while a long-term study of changes in acid-base chemistry in four gaged SNP streams has been in operation since 1979. Statistical analysis of the long-term record of mean daily discharge and weekly streamwater chemical composition was performed to identify the dominant processes that control episodic acidification of SNP streams; an objective hydrological separation technique was used to determine whether each weekly sample was taken during antecedent baseflow or stormflow conditions. Using this technique, more than 100 “stormflow/baseflow pairs” were identified in the long-term (1980–1992) record for White Oak Run from which episodic chemical changes were estimated. Statistical analysis of the resultant data suggested that mean episodic depressions in ANC have increased by nearly a factor of two in White Oak Run since the first outbreak of forest defoliation by the gypsy moth caterpillar during the summer of 1990.